I don't worship 3.x, but it's the Swiss army knife I need
It was a shock for me as well when I switched from TB2 to TB3: I was quite taken aback by the "augmented" features, and not very cool about those new default settings such as IMAP synchronization (I hurried to disable this as soon as I realized what was going on). I use only IMAP e-mail (multiple accounts), but I also need PGP/GPG encryption & signature, plus LDAP support for my contacts, and Linux compatibility. On the top of this, there are quite a few TB extensions I like and use, even though they're not essential. Given what I need, the only viable alternative I know of is Evolution, but I tried it some time and had problems whith it, and it doesn't have cool extensions I can find for Thunderbird. Even though TB2 was heaven for me, TB3 works at least well enough for me so I can stick to it for now, but I do hope they make dramatic improvements for the next versions!